The invention relates to a process for the preparation of granules by feeding a liquid material into a fluidized bed of solid nuclei, upon which the nuclei grow by solidification of the liquid material on them, and removal of the granules thus formed from the fluidized bed.
Such a process is known from, for instance, The Soviet Chemical Industry 4 (1972) No. 7, pp. 456-458, and 5 (1973), No. 4, pp. 265-267, and from Verfahrenstechnik 9 (1975) No. 2, pp. 59-64.
In these known processes, the liquid material, for example in the form of a solution, melt or suspension, is with the aid of a gas sprayed to droplets, which on the fluidized nuclei solidify to form granules of the desired size. In order for the granulation process to proceed well, it is necessary that the surface of the grown nuclei solidifies sufficiently quickly to prevent agglomeration of individual particles. It must be ensured, therefore, that the sprayed liquid material crystallizes quickly, and that the water present evaporates quickly. In the known processes this is achieved by spraying the liquid material to fine droplets, or even atomizing it. Of course, the rule here is that the more water is to be evaporated, the finer the atomization should be. The size of the droplets obtained in spraying is mainly determined by the pressure and the quantity of the spraying gas, the general rule being that as this pressure and quantity are higher, the drops obtained are smaller. It is therefore common practice to apply a spraying gas with a fairly high feed pressure, for example 1.5 bar or more, as described in, inter alia, Khim. Naft. Mashinostr. (1970 ) No. 9, pp. 41-42, and in U.S. Pat. No. 4,219,589, to obtain droplets of relatively small average diameter.
A disadvantage of these known processes is that for spraying the liquid material to droplets a large amount of gas of high pressure is required, which, of course, is accompanied by a high energy consumption. A possible explanation of this is that the liquid material contacts the gas as a jet. The jet is broken up into drops by the gas stream pealing the outer layers from the jet. This process continues along some distance in downstream direction. In this process, the gas is necessarily slowed down, resulting in progressively poorer atomization. In order still to achieve a sufficiently fine atomization, therefore, a high mass ratio of high-energetic gas to liquid material must be applied. It has been found that for sufficiently fine atomization of all liquid material this ratio should generally be higher than 1. Admittedly, it is possible to conduct this known spraying method with a lower mass ratio of gas to liquid material, but then a gas stream of very high feed pressure must be applied, for example more than 4 bar, which of course implies a very high energy consumption.
In principle, fine atomization can be achieved also by hydraulically spraying the liquid material, at very high liquid feed pressures (tens of bars). The energy consumption is then lower than in the previously described processes, but this method has the disadvantage of extreme wear on the spraying device. Moreover, serious agglomeration of nuclei in the fluidized bed is found to occur with this spraying method.
According to another known process, which is described in, for example, GB Nos. A 2,019,302 and GB A 2,075,908, in a fluidized-bed granulation process the liquid material is with the aid of a hydraulic sprayer divided into relatively large drops, which are subsequently finely atomized with the aid of a powerful gas stream. To this end, the liquid material is sprayed upward into the fluidized bed of nuclei via a sprayer provided with two concentric channels, the liquid material being supplied through the inner channel and the drops being contacted, shortly after leaving this channel, with a powerful gas stream supplied through the outer channel. By the powerful gas stream, above the sprayer a zone is created in the fluidized bed with a very low concentration of nuclei, the so-called rarefied zone, into which nuclei are sucked from the fluidized bed and moistened with droplets of liquid material. Although in this known process the required amount of high-energetic gas is lower than in the processes mentioned in the introduction, this amount is found to be still quite substantial. It has been found that for good atomization of all liquid material the required mass quantity of high-energetic gas must be more than 50% of the mass quantity of liquid material. Moreover it has been found that in the known processes, particularly during granulation of urea, a high formation of dust occurs. This implicates lost of production and serious environmental problems. Admittedly, during the granulation of urea this dust formation can be diminished by adding a large amount of formaldehyde to the liquid feed. This of course implies very high costs.
An essential part of all these known processes is, clearly, that the liquid material is converted to more or less fine droplets, which, as argued, is accompanied by a fairly high energy consumption and dust emission.